of the tenth or eleventh magnitude, and the only way in which they can be detected, therefore, is to compare the star-charts of different parts of the heavens with the heavens themselves, night after night. Should any point of light be observed not marked on the chart, it is immediately watched, and if any motion is detected, the amount and direction are determined. Some idea of the diligence and patience required for this branch of observation may be gathered from Fig. 27, which is the reduction of a part of one of M. Chacornac's ecliptic charts. The faint diagonal line shows the path of a minor planet across that portion of the heavens represented. 285. With regard to the cause for the existence of these little bodies, it was long ago suggested that they may be fragments of a larger planet destroyed by contact with some other celestial body. A closer investigation of the character and distribution of their orbits has, however, of late thrown much doubt on this view Yet the remark of D'Arrest should not be lost sight of: "One fact," he says, 66 appears, above all, to confirm the idea of an intimate relation between all the minor planets: it is, that if their orbits are figured under the form of material rings, these rings will be found so entangled that it would be possible, by means of one among them, taken at hazard, to lift up all the rest." 286. Pallas has been supposed, from its hazy appearance, to be surrounded by a dense atmosphere, and this may also be the case with the others, as their colours are not the same. There are also evidences that some among them rotate on their axes like the larger planets. LESSON XXIII.-Comets: their Orbits. Short-period Comets. Head, Tail, Coma, Nucleus, Jets, Envelopes. Their probable Number and Physical Constitution. 287. We have seen that round the white-hot Sun cold or cooling solid bodies, called planets, revolve; that be cause they are cold they do not shine by their own light; that they perform their journeys in almost the same plane; that the shape of their orbit is oval or elliptical; and that they all move in one direction,—that is, from west to east. But these are not the only bodies which revolve round the Sun. In addition to them there are bodies called comets, consisting of clouds of stones or meteorites, which do shine by their own light in consequence of collisions between the stones. These perform their journeys round the Sun in every plane, in orbits which in some cases are so elongated that they can scarcely be called elliptical, and—a further point of difference-while some revolve round the Sun in the same direction as the planets, others revolve from east to west. 288. The orbit of a comet is generally best represented by what is called a parabola; that is, an infinitely long ellipse, which latter, like a circle, is a closed curve—whereas the parabola may be regarded as an open one (Chap. IX.). In the case of a comet whose whole orbit has been watched, we know that the orbit is elliptical. In the case of those with parabolic orbits, we know not whence they come or whither they are going, and therefore we cannot say whether they will return or not. There are some comets whose return may be depended upon and calculated with certainty. Here is a list of some of them : We may add the comet known as Biela's, with a period of 6 years, but which, there is reason to believe, has ceased to exist as a comet. (See Art. 297). 289. These are called short-period comets. Cf the long-period comets we may instance the comets of 1858, 1811, and 1844, the times of revolution of which have been estimated at 2,100, 3,000, and 100,000 years respectively. 290. From the table that we have given it will be seen how the distance of these erratic bodies from the Sun varies in different points of the orbit. Thus Encke's comet is ten times nearer the Sun in perihelion than at aphelion. Some comets, whose aphelia lie far beyond the orbit of Neptune, approach so close to the Sun as almost to graze its surface. Sir Isaac Newton estimated that the comet of 1680 approached so near the Sun that its temperature was 2,000 times that of red-hot iron: at the nearest approach it was but onesixth part of the sun's diameter from the surface. The comets of 1843 and 1882 also approached very close to the Sun, and both were visible in broad daylight. 291. We next come to what a comet is like. In Fig. 28 we give a repre sentation of Donati's comet, visible in 1858, which will make a general description clear. The brighter part of the comet is called the head, or coma, and sometimes the head contains a brighter portion still, called the nucleus. The tail is the dimmer part flowing from the head, and, as observed in different comets, it may be long or short, straight or curved, single, double, or multiple. The comet of 1744 had six tails, that of 1823 two. In some comets the tail is entirely absent. Both head and tail are so transparent that all but the faintest stars are easily seen through them. In 1858 the bright star Arcturus was seen through the tail of Donati's comet at a place where the tail was 90,000 miles in diameter. 292. In olden times, when less was known about comets, they caused great alarm-not merely superstitious terror, which connected their coming with the downfall of a king or the outbreak of a plague, but a real fear that they would dash this little planet to pieces should they come into contact with it. Modern science teaches us that in the great majority of instances the mass of the comet is so small that we need not be alarmed; indeed, there is good reason to believe that on June 30, 1851, we did actually pass through the tail of the glorious comet which then became so suddenly visible to us. There is another fact too which teaches us the same thing. In 1779 a comet approached so close to Jupiter that it may actually have got entangled among the satellites of that planet, but the satellites all the time pursued their courses as if the comet had never existed. This, however, was by no means the case with the comet; it was thrown entirely out of its course, and has changed its orbit from one bringing it back to the Sun every 5 years to one in which it has never since been visible to terrestrial observers. 293. The number of comets recorded from the earliest times, beginning with the Chinese annals, to our own, is nearly 900, but the number observed at present is much greater than formerly, as many telescopic ones are now recorded, whereas the old chronicles tell us only of those in bygone times which were brilliant enough to attract general attention, and to give rise to the most gloomy forebodings. It is worthy of remark that in the year of the Norman invasion, 1066, a fine comet with three tails appeared, which in the Norman chronicle is given as evidence of William's divine right to invade this country. It has been estimated that there may be many millions of comets belonging to our system, and perhaps passing between this and other systems. We see but few of them, because those only are visible to us which are well placed for observation when they pass the Earth in their journey to or from perihelion, while there may be thousands which at their nearest approach to the Sun are beyond the orbit of Neptune. 294. We have already stated that these bodies consist of clouds of meteorites. Now, when they are far away from the Sun, the collisions are few, and their light is dim, and we observe them in our telescopes as round misty bodies, moving very slowly, say a few yards in a second, in the depths of space. Gradually, as the comet approaches the Sun, and the collisions increase (for, as we shall see in Chap. IX., the nearer any body, be it planet or comet, gets to the Sun, the faster it travels), the Sun's action begins to be felt, the comet gets hotter and becomes visible to the naked eye. A violent action commences; jets of meteoric vapours burst forth from the coma towards the Sun, and are instantly driven back again. The jets rapidly change their position and direction, and a tail is formed consisting of the finer particles resulting from collisions, and rendered visible in part by Sunlight, in part by their own incandescence. The tail-forming materials are most likely driven from the Sun by electrical repulsion. The variety in the forms of these appendages is now thought to be caused by differences in chemical constitution. The lightest and |